Retrofitting of the existing external RC beam-column joints with insufficient development length of longitudinal rebars by anchor dowel FRP composites

Beam-column joints are one of the most important members of the reinforced concrete structures that are responsible for transferring existing loads. These joints require some criteria such as the development length of the beam's and column's rebars for proper operation. Lack of sufficient development length reduces the capacity of the structure and thus increases its deformation. In this study, three specimens of external reinforced concrete beam-column joints were tested on a real scale with applying lateral loads. The dimensions of beams and columns are 300 and 300 mm. Samples include a control sample with insufficient development length of longitudinal rebars of the beam (at joint area), a sample with sufficient development length through end mechanical restraint, and another sample with insufficient development length and retrofitting with dowel anchor carbon FRP composites (to improve the beam-Column joint). For making FRP anchors, columns should be pierced and dowels pass through this hole and they connect to concrete elements surface with proper epoxy adhesive. In all samples, lateral displacement load cycle diagrams (hysteresis diagram), failure modes, ductility coefficient, and stiffness were extracted and compared with each other. The results of this study showed that lack of sufficient development length leads to severe damage in the joint and plastic hinge is not formed, and energy dissipation and crack formation is occurring in the joint; while in retrofitted and restrained sample improvement of seismic performance, formation of the plastic joint in the beam and effects of sufficient development length is observed and energy dissipates in that area. Pinching in the hysteresis diagram of the control specimen is observed and maximum load capacity is much less than the others. end restraint of the rebar and reinforcement of the specimen with FRP increases the lateral bearing capacity of the joint by 53% compared to the control specimen. Also, the residual capacity increases by about 84% and 73% respectively at the end of the test (corresponding to 6% drift). The ductility of the sample with end restraint and the sample reinforced with dowel anchor FRP composites are 34% and 19% higher than the control sample, respectively. Insufficient development length also reduces the stiffness of the control sample by up to 45% compared to the restrained and retrofitted sample. Another result of insufficient development length of rebars of the beam is the angle between beam and column. In the fixed connections, this angle should remain constant while in the control specimen, this angle changes dramatically. By retrofitting the connection with FRP composites or restraining the rebars, this angle varies much less. The results of three tested samples showed that strengthening the connection with anchor dowel FRPs can effectively fix the lack of development length of rebars in the beams and increase the load capacity, stiffness, and ductility of the connection.